Many reports have hitherto been made on condensed heterocyclic compounds, for example, on 1,3-dihydrobenzo[c]thiophene, 1,3-dihydronaphtho[2,3-c]thiophene, and their analogous derivatives (for example, in A. R. Katrizky and A. J. Boulton, Advances in Heterocyclic Chemistry, vol. 10, Academic Press, New York (1969)). A generally known example of the synthesis method concerned is a method to produce 1,3-dihydrobenzo[c]thiophene or 1,3-dihydronaphtho[2,3-c]thiophene, comprising the ring-closure of o-dibromoxylene, o-dichloroxylene, 2,3-bisbromo-methylnaphthalene, or the substitution products thereof using sodium sulfide (see Recl. Trav. Chim. Pays-Bas, vol. 87(10), p. 1006, 1968). Further, reports have also been made on the ring-closure reaction of the above-described dihalogeno compounds using lithium sulfide and the reduction reaction of thiophthalic acid anhydride.
However, 1,3-dihydrobenzo[c]thiophene,1,3-dihydronaphtho[2,3-c]thiophene, and their analogous derivatives are deficient because they are highly reactive and cannot be stored in air at room temperature (about 20.degree.-30.degree. C.) for a long period of time (an oxidative dehydrogenation reaction being spontaneously and incompletely induced to give rise to blackening as a result of undesired oligomerization or polymerization).
Accordingly, for incorporating a substituent into an aromatic ring, a desired substituent is previously incorporated into a precursor, then the above-described dihalogeno compound is produced, and the ring-closure reaction is carried out according to the above-described method and the like to produce a 1,3-dihydrobenzo[c]thiophene or 1,3-dihydronaphtho[2,3-c]thiophene derivative with a substituent (see JP-A-2-308847, the term "JP-A" as used herein means an unexamined published Japanese patent application").
EP291269-A1 describes a 1,3-dihydrobenzo[c]thiophene derivative having a sulfonamide group as an example of a compound with a substituent, but the publication does not disclose any specific production process thereof. Among condensed heterocyclic compounds with a sulfonic acid group, JP-A-2-252727 describes a compound having a benzo[c]thiophene (or alternatively called isothianaphthene) in which condensed benzene ring is conjugated with carbon atoms in the 1- and 3-positions, but the publication does not disclose any specific production process thereof nor the properties thereof. Further, there is no report of a condensed heterocyclic compound with both a sulfonic acid group and a dihydrothiophene structure, for example, a sulfonic acid-substituted compound of 1,3-dihydrobenzo[c]thiophene, 1,3-dihydronaphtho[2,3-c]thiophene, or a derivative thereof.